1. Field of the Invention
The present invention relates to image-capturing apparatuses, such as video cameras and electronic still cameras, and, more specifically, relates to optical apparatuses using electroactive polymer actuators. The invention relates particularly to a diaphragm device (light intensity adjustment device) disposed in an optical path in a lens barrel included in an image-capturing apparatus.
2. Description of the Related Art
Recently, the size of video cameras and electronic still cameras has been significantly reduced. At the same time, various small camera units for mobile phones have been developed. Thus, the need for reducing the size of lens units for capturing images has increased.
Conventionally, such video cameras and electronic still cameras have typically used electromagnetic motors as actuators for driving movable lenses and light adjustment diaphragms for automatic focus, zooming, hand-shake compensation, and light adjustment. Inside the lens barrel, an electromagnetic motor is provided. There is a need in reducing the size of the space occupied by the motor in the lens barrel since the size of lenses have been decreased along with the reduction in image size.
An electromagnetic motor includes a magnet and a coil. An electromagnetic motor uses the force that is generated in accordance with Fleming's rules at the magnet and the coil by applying an electric current to the coil disposed in the magnetic field of the magnet, as a driving force. To reduce the size of an actuator, the generated driving force per volume must be increased. Thus, to increase the driving force, the electric current applied to the electromagnetic actuator and the magnetic field generated at the magnet must be increased.
More specifically, the coil diameter must be increased to enable a large current to be applied; the number of coil windings must increased; or the size of the magnet must be increased to increase the density of magnetic flux. However, all of these approaches cause an increase in the size of the motor and, therefore, are not suitable for the recent trend in small image-capturing devices.
When driving a great load with a small driving force, typically, a combination of a deceleration mechanism and mechanical leverage is used. However, when a deceleration mechanism is used, the motor will be driven at a speed higher than the driving speed of the load. Therefore, there is a problem in that noise caused by, for example, driving the motor and operating the gears of the deceleration mechanism is generated. There is also a problem in that the addition of the deceleration mechanism and a mechanical component for supplying leverage causes an increase in size and cost. Another problem is that durability performance is reduced because a mechanical sliding member is provided.
Recently, active research of a polymer material that can be greatly distorted and that has a great generative force per volume has been carried out with the aim of application to the development of artificial muscles. An actuator has been developed by applying such technology. Such actuator does not require a deceleration mechanism because the distortion of the material is several orders of magnitude greater than known piezoelectric material, such as piezoelectric zirconate titanate (PZT), and the distortion of the material is directly transmitted to the load as a driving force.
Such materials includes electroactive polymers. Electroactive polymer typically include dielectric elastomer, ferroelectric polymer, liquid-crystal elastomer, and electrostrictive polymer (Y. Bar-Cohen, Ed., “Electro Active Polymers (EAP) as Artificial Muscles: Reality Potential and Challenges.” 2nd Edition, pp. 22-P31, Bellingham, Wash.: SPIE Press, 2004).
In particular, for dielectric elastomer, there are acrylic and silicon dielectric elastomers. Acrylic dielectric elastomer has been receiving attention because the distortion of some types of acrylic dielectric elastomer is 380% or more. The generative force per volume of acrylic dielectric elastomer is a couple of orders of magnitude greater than a conventional electromagnetic motor. Thus, it is expected that the volume of the actuator can be reduced to less than one tenth of that of a conventional electromagnetic motor. Since the generative force and distortion are great, a deceleration mechanism and mechanical leverage, such as those described above, are not required. Therefore, a quiet and durable actuator can be produced.
The operation principle of this dielectric elastomer is described below for a flat dielectric elastomer.
A flat dielectric elastomer interposed between two electrodes is compressed (Maxwell stress) in the direction of the electric field by the electrostatic force generated between the electrodes when a voltage is applied to the electrodes, and, simultaneously, a pressure P is generated in a manner such that the pressure P spreads out in a direction orthogonal to the direction of the electrical field. The pressure P can be represented by Expression 1 (Y. Bar-Cohen, Ed., “Electro Active Polymers (EAP) as Artificial Muscles: Reality Potential and Challenges.” 2nd Edition, pp. 535-539, Bellingham, Wash.: SPIE Press, 2004). This pressure P is used as a force for driving the actuator. As apparent from Expression 1, to increase the driving force, the dielectric permittivity of the material may be increased, the distance between the electrodes may be decreased, and the driving voltage may be increased.P=εrε0(V/t)2  (1)where εr represents the relative permittivity of a film, ε0 represents the dielectric permittivity of a vacuum (=8.85×10-12 F/m), V represents the voltage between the electrodes, and t represents the distance between the electrodes.
The relationship between the displacement of the film and load can be represented by Expression 2 described below (Y. Bar-Cohen, Ed., “Electro Active Polymers (EAP) as Artificial Muscles: Reality Potential and Challenges.” 2nd Edition, pp. 535-539, Bellingham, Wash.: SPIE Press, 2004):Δl=l(0.5P−F/wt)/Y  (2)where Δl represents the displacement (stretching of film) of the actuator in the extraction direction the force, l represents the initial length of the film, P represents the generated pressure (Expression 1), F represents load, w represents the width of the film, t represents the thickness of the film, and Y represents Young's modulus.
A material such as elastic carbon is provided on an electrode above the dielectric elastomer. Acrylic dielectric elastomer (VHB4910 manufactured by 3M) and silicon dielectric elastomer are commercially available.
U.S. Pat. No. 6,891,317 describes a cylindrical actuator constructed by wrapping such a dielectric elastomer film around a compression coil spring. Depending on the structure of the electrodes, this cylindrical actuator functions as a one-dimensional linear actuator that extends and contracts in the axial direction or a two-dimensional flexion actuator in which the end portion of the cylinder bends. A push-pull actuator constructed of two cylindrical actuators is described in “Science American” (p. 58, October 2003). This compression coil spring applies a prestrain to the dielectric elastomer film in the peripheral direction and the axial direction. The strength of the dielectric elastomer film against damage by electrostatic discharge increases when prestrain is applied. Thus, by increasing the strength of the dielectric elastomer film by applying prestrain, the voltage applied to the film can be increased, and, consequently, the driving force can be increased. As a result, the size of the actuator can be reduced and the reliability of the actuator is improved. The strength against damage by electrostatic discharge of a silicon dielectric elastomer film is 110 to 350 MV/m and of acrylic dielectric elastomer film is 125 to 440 MV/m (J D W. Madden, “Artificial Muscle Technology: Physical Principles and Naval Prospects.” IEEE Journal of Oceanic Engineering, Vol. 29, No. 3, July 2004).
U.S. Pat. No. 6,809,462 discusses the application of dielectric elastomer to a displacement sensor by detecting the change in electrical properties, such as capacitance and resistance of the dielectric elastomer caused by deformation of the dielectric elastomer. However, methods of detecting changes in capacitance and other values is not limited, and other methods, such as RCA radiofrequency resonator, are well known. Thus, a known detection circuit can be used to detect changes in electrical properties, such as resistance. An actuator using a piezoelectric element provided as a single unit with a sensor is also well known.
Next, an overview of the mechanical structure of a diaphragm device (light adjustment device) using a known actuator (e.g., electromagnetic actuator, such as a meter) will be described (refer to FIGS. 3, 6, and 9).
FIGS. 3A to 3C illustrate only the main components of a diaphragm device according to a first conventional example. FIG. 3A is a front view with a closed diaphragm; FIG. 3B is a front view with an open diaphragm; and FIG. 3C is a side view.
A first movable diaphragm blade 1 has a lateral oblong hole 1-a and longitudinal oblong holes 1-b and 1-c. A second movable diaphragm blade 2 has a lateral oblong hole 2-a and longitudinal oblong holes 2-b and 2-c. At both ends of a movable arm 3, pins 3-a and 3-b provided. A meter (driving source) 4 and fixed guide pins 5, 6, 7, and 8 are also provided. Light passes through an imaginary effective diameter 9.
The diaphragm device having the above-described structure operates as described below.
The pin 3-a of the arm 3 is engaged with the lateral oblong hole 1-a of the diaphragm blade 1. The longitudinal oblong holes 1-b and 1-c of the diaphragm blade 1 are engaged with the fixed guide pins 5 and 6, respectively.
The pin 3-b of the arm 3 is engaged with the lateral oblong hole 2-a of the diaphragm blade 2. The longitudinal oblong holes 2-b and 2-c of the diaphragm blade 2 are engaged with the fixed guide pins 7 and 8, respectively.
The meter 4 generates a rotationally reciprocating driving force to rotate the arm 3 around axis X in a reciprocating manner. When the arm 3 is rotated to the right (FIG. 3A), the diaphragm blade 1 moves upward and the diaphragm blade 2 moves downward, covering the effective diameter to close the diaphragm.
When the arm 3 is rotated to the left (FIG. 3B), the diaphragm blade 1 moves downward and the diaphragm blade 2 moves upward, being moved out of alignment with the effective diameter to open the diaphragm.
FIGS. 6A to 6C illustrate only the main components of a diaphragm device according to a second conventional example. FIG. 6A is a front view with a closed diaphragm; FIG. 6B is a front view with an open diaphragm; and FIG. 6C is a side view. A hole 11-a and an oblong hole 11-b are formed in a first movable diaphragm blade 11. A hole 12-a and an oblong hole 12-b are formed in a second movable diaphragm blade 12. A pin 13-a is provided at an end of a movable arm 13. A meter (driving source) 14 and fixed pins 15 and 16 are also provided. Light passes through an imaginary effective diameter 19.
The diaphragm device having the above-described structure operates as described below.
The pin 13-a of the arm 13 is engaged with the oblong hole 11-b of the diaphragm blade 11 and the hole 12-b of the diaphragm blade 12. The oblong hole 11-a of the diaphragm blade 11 and the hole 12-a of the diaphragm blade 12 are engaged with fixed pins 15 and 16, respectively. The meter 14 generates a rotationally reciprocating driving force to rotate the arm 13 around axis X in a reciprocating manner. When the arm 13 is rotated to the left (FIG. 6A), the diaphragm blade 11 rotates to the left around the pin 15 and the diaphragm blade 12 rotates to the right around the pin 16, covering the effective diameter to close the diaphragm. When the arm 13 is rotated to the right (FIG. 6B), the diaphragm blade 11 rotates to the right around the pin 15 and the diaphragm blade 12 rotates to the left around the pin 16, being moved out of alignment with the effective diameter to open the diaphragm.
FIGS. 9A to 9C illustrate only the main components of a diaphragm device according to a third conventional example. FIG. 9A is a front view with a closed diaphragm; FIG. 9B is a front view with an open diaphragm; and FIG. 9C is a side view.
A hole 21-a and an oblong hole 21-b are formed in a first movable diaphragm blade 21. A hole 22-a and an oblong hole 22-b are formed in a second movable diaphragm blade 22. A hole 23-a and an oblong hole 23-b are formed in a third movable diaphragm blade 23. A hole 24-a and an oblong hole 24-b are formed in a fourth movable diaphragm blade 24. A hole 25-a and an oblong hole 25-b are formed in a fifth movable diaphragm blade 25. A hole 26-a and an oblong hole 26-b are formed in a sixth movable diaphragm blade 26. Six pins 27-a1, 27-a2, 27-a3, 27-a4, 27-a5, and 27-a6 are provided on the ring portion of the rotary member 27. An oblong hole 27-b is provided on the lever portion of the rotary member 27. A pin 28-a is provided at the edge of a movable arm 28. A meter (driving source) 30 and fixed pins 31, 32, 33, 34, 35, and 36 are also provided. Light passes through an imaginary effective diameter 29.
The diaphragm device having the above-described structure operates as described below.
The meter 30 generates a rotationally reciprocating driving force to rotate the arm 28 around axis X in a reciprocating manner. Since the pin 28-a of the arm 28 is engaged with the oblong hole 27-b of the rotary member 27, when the arm 28 rotates to the right around axis X, the rotary member 27 rotates to the left around the optical axis O, whereas, when the arm 28 rotates to the left around axis X, the rotary member 27 rotates to the right around the optical axis O. The six pins 27-a1 to 27-a6 provided on the ring portion of the rotary member 27 are engaged with the oblong holes 21-b to 26-b, respectively, of the diaphragm blades 21 to 26. The holes 21-a to 26-a are engaged with fixed pins 31 to 36, respectively. In this way, when the rotary member 27 rotates to the left around the optical axis O, the diaphragm blades 21 to 26 rotate to the right around the fixed pins 31 to 36, covering the effective diameter to close the diaphragm. When the rotary member 27 rotates to the right around the optical axis O, the diaphragm blades 21 to 26 rotate to the left around the fixed pins 31 to 36, being moved out of alignment with the effective diameter to open the diaphragm.
In other words, when the arm 28 rotates to the right (FIG. 9A), the diaphragm blades 21 to 26 cover the effective diameter to close the diaphragm, whereas, when the arm 28 rotates to the left (FIG. 9A), the diaphragm blades 21 to 26 are moved out of alignment with the effective diameter to open the diaphragm.
The diaphragm device according to a conventional example includes a meter powered by an electromagnetic force as a driving source.
As described above, with a known diaphragm device using an electromagnetic actuator, the volume of the actuator is great. For example, the volume of the actuator is about φ8×8=402 mm3, and the area viewed from the direction of the optical axis as large as the diameter of an open diaphragm. Therefore, the actuator has been an obstacle to reducing the size of the diaphragm device.
The mechanical structure of the diaphragm device according to the above-described conventional examples, the meter is disposed in a space other than where the effective diameter (i.e., opening) and other members (i.e., arm, rotary member, and diaphragm blade) are disposed. Therefore, the size of the meter was a great factor determining the size and weight of the diaphragm device. Since the meter accommodates mechanisms and members for converting an electromagnetic force into mechanical movement, the number of components included in the device was great, and the components were expensive.